Place a rack in the center of your oven and preheat the oven to 350 degrees F. Line an 8x8-inch pan with parchment paper so that the paper overhangs two of the sides like handles, then lightly coat with baking spray.
In a large mixing bowl, beat the butter and dark brown sugar together on high speed for 2 minutes, until completely smooth and well combined. Beat in the eggs one at a time. Beat in the milk and vanilla, then the cinnamon and salt.
Sprinkle the baking powder, baking soda, whole wheat pastry flour, and all purpose flour over the top. With a spatula or wooden spoon, stir by hand just until the flour disappears. Scrape the batter into the prepared pan and smooth the top. Bake the blondies for 35-40 minutes, just until the middle is set and a toothpick inserted in the center comes out clean. Do not overbake! Let cool in the pan for 5 minutes.
For the topping, in a small bowl, stir together the sugar and cinnamon. Brush the top of the blondies with 1 tablespoon melted butter, then sprinkle the cinnamon-sugar mixture over the top. It will seem like too much, but use it all. Using the parchment paper handles, gently lift the blondies from the pan and transfer to a wire rack to cool completely. Once cool, slice and enjoy.
Store leftover Cinnamon Blondies in an airtight container in the refrigerator or at room temperature for up to 5 days. Freeze for up to 3 months. Let thaw overnight in the refrigerator, then bring to room temperature prior to serving.